Circuit Note
CN-0288
Devices Connected/Referenced
Circuits from the Lab® reference designs are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0288.
AD598
LVDT Signal Conditioner
AD8615
Precision, 20 MHz, CMOS, Single RRIO
Operational Amplifier
AD7992
2-Channel, 12-Bit ADC with I2C Compatible
Interface in 10-lead MSOP
LVDT Signal Conditioning Circuit
This circuit uses the AD598 LVDT signal conditioner that contains
a sine wave oscillator and a power amplifier to generate the
excitation signals that drive the primary side of the LVDT. The
AD598 also converts the secondary output into a dc voltage. The
AD8615 rail-to-rail amplifier buffers the output of the AD598 and
drives a low power 12-bit successive approximation analog-todigital converter (ADC). The system has a dynamic range of 82 dB
and a system bandwidth of 250 Hz, making it ideal for precision
industrial position and gauging applications.
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
CN-0288 Circuit Evaluation Board (EVAL-CN0288-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 is a complete adjustment-free linear
variable differential transformer (LVDT) signal conditioning
circuit. This circuit can accurately measure linear displacement
(position).
The signal conditioning circuitry of the system consumes only
15 mA of current from the ±15 V supply and 3 mA from the +5 V
supply, making this ideal for remote applications. The circuit can
operate a remote LVDT from up to 300 feet away, and the output
can drive up to 1000 feet.
The LVDT is a highly reliable sensor because the magnetic core
can move without friction and does not touch the inside of the
tube. Therefore, LVDTs are suitable for flight control feedback
systems, position feedback in servomechanisms, automated
measurement in machine tools, and many other industrial and
scientific electromechanical applications where long term reliability
is important.
This circuit note discusses basic LVDT theory of operation and the
design steps used to optimize the circuit shown in Figure 1 for a
chosen bandwidth, including noise analysis and component
selection considerations.
+15V
EXCITATION (CARRIER)
VA
3
2
11
20
AMP
+5V
OSC
+5V
AD598
17
33Ω
VB
10
A–B
A+B
VOUT
FILTER
AMP
16
3kΩ
0.01µF
AD8615
VIN1
AD7992
2.7nF
SDA
SCL
ALERT
1
11426-001
E-100
ECONOMY SERIES LVDT
–15V
Figure 1. LVDT Signal Conditioning Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. A
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CN-0288
Circuit Note
CIRCUIT DESCRIPTION
Theory of Operation
An LVDT is an absolute displacement transducer that converts
a linear displacement or position from a mechanical reference
(or zero) into a proportional electrical signal containing phase
(for direction) and amplitude information (for distance). The
LVDT operation does not require electrical contact between the
moving part (probe or core rod assembly) and the transformer.
Instead, it relies on electromagnetic coupling. For this reason,
and because they operate without any built-in electronic circuitry,
LVDTs are widely used in applications where long life and high
reliability under severe environments are a required, such
military and aerospace applications.
For this circuit, the E-100 Economy Series LVDT sensor from
Measurement Specialties™, Inc. was used with the AD598. With
a linearity of ±0.5% of full range, the E Series is suitable for
most applications with moderate operation temperature
environments.
The AD598 is a complete, LVDT signal conditioning subsystem.
It converts the transducer mechanical position of LVDTs to a
unipolar dc voltage with a high degree of accuracy and
repeatability. All circuit functions are included on the chip.
With the addition of a few external passives components to
set frequency and gain, the AD598 converts the raw LVDT
secondary output to a scaled dc signal.
The AD598 contains a low distortion sine wave oscillator to
drive the LVDT primary. The frequency of the sine wave is
determined by a single capacitor and can range from 20 Hz to
20 kHz with amplitudes from 2 V rms to 20 V rms.
The LVDT secondary output consists of two sine waves that drive
the AD598 directly. The AD598 operates upon the two signals,
dividing their difference by their sum and producing a scaled
unipolar dc output. Previous LVDT conditioners synchronously
detect this amplitude difference and convert its absolute value to
a voltage proportional to position. This technique uses the primary
excitation voltage as a phase reference to determine the polarity
of the output voltage. There are a number of problems associated
with this technique. They include:
•
•
•
Producing a constant amplitude, constant frequency
excitation signal
Compensating for LVDT primary to secondary phase shifts
Compensating for these shifts as a function of temperature
and frequency
The ratiometric principle upon which the AD598 operates
requires that the sum of the LVDT secondary voltages remains
constant with LVDT stroke length. Although LVDT manufacturers
generally do not specify the relationship between VA + VB and
stroke length, it is recognized that some LVDTs do not meet this
requirement. In these cases, a nonlinearity results. However, the
majority of available LVDTs do in fact meet these requirements.
Component Selection
The design procedure for the dual supply operation (±15 V)
found in the AD598 data sheet was followed to set the excitation
frequency to 2.5 kHz, system bandwidth to 250 Hz, and an
output voltage from 0 V to 5 V.
It is normal for the AD598 internal oscillator to produce a small
amount of ripple that feeds through to the output. A passive
low-pass filter is used to reduce this ripple to the required level.
When selecting capacitor values to set the bandwidth of the system,
a trade-off is involved. Choosing smaller capacitors give higher
system bandwidth but increase the amount of output voltage
ripple. The ripple can be reduced by increasing the shunt
capacitance across the feedback resistor used to set the output
voltage level; however, this also increases phase lag.
The AD8615 operational amplifier buffers the output of the
AD598, which ensures that the AD7992 ADC is driven by a low
impedance source (high source impedances significantly affect
the ac performance of the ADC).
The low-pass filter between the output of the AD598 and the
input of the AD8615 serves two purposes:
•
•
It limits the input current to the AD8615
It filters the output voltage ripple.
The AD8615 has internal protective circuitry that allows voltages
exceeding the supply to be applied at the input. This is important
because the output voltage of the AD598 can swing ±11 V with
±15 V supplies. As long as the input current is limited to less
than 5 mA, higher voltages can be applied to the input. This is
primarily due to the extremely low input bias current of the
AD8615 (1 pA) which allows the use of larger resistors. The
use of these resistors adds thermal noise, which contributes to
the overall output voltage noise of the amplifier.
The AD8615 is an ideal amplifier to buffer and drive the input
of the AD7992 12-bit SAR ADC because of its input overvoltage
protection, and its ability to swing rail-to-rail at both the input
and output.
The AD598 eliminates all of these problems. The AD598 does
not require a constant amplitude because it works on the ratio
of the difference and sum of the LVDT output signals. A constant
frequency signal is not necessary because the inputs are rectified
and only the sine wave carrier magnitude is processed. There is
no sensitivity to phase shift between the primary and the LVDT
outputs because synchronous detection is not employed.
Rev. A | Page 2 of 6
Circuit Note
CN-0288
Noise Analysis
With all signal condition components selected, the amount of
resolution needed to convert the signal must be determined.
As in most noise analyses, only the key contributors need to be
identified. Noise sources combine in an rss manner; therefore,
any single noise source that is at least three-to-four times larger
than any of the others dominates.
In the case of the LVDT signal conditioning circuit, the dominant
source of the output noise is the output ripple of the AD598.
The other sources of noise (resistor noise, input voltage noise,
and output voltage noise of the AD8615) are significantly
smaller in comparison.
The output voltage ripple of the AD598 is 0.4 mV rms with a
0.39 µF capacitor value and with a 10 nF shunt capacitor across the
feedback resistor shown in Figure 2. Note that these components
and related pin connections are not shown in the simplified
schematic in Figure 1; however, details can be found in the
AD598 data sheet.
The total output dynamic range of the system can be calculated
by dividing the full-scale output signal (5 V) by the total output
rms noise (0.4 mV rms) and converting it to decibels, yielding
approximately 82 dB.
Dynamic Range = 20 log(5 V/0.4 mV) = 82 dB
The AD7992 is a good candidate for this application because it
has 12-bit resolution and a sampling rate of 188 kSPS per channel
when used with a 3.4 MHz serial clock.
Test Results
Using a Measurement Specialties, Inc. E-100 Economy Series
LVDT connected to J3 and using a digital oscilloscope to
monitor the output of the AD598 found on J6 on the EVALCN0288-SDPZ evaluation board, the actual output ripple found
was 6.6 mV p-p, as is shown in Figure 3.
1000
1
10
2.5kHz, CSHUNT = 0nF
CH1 2.0mV
2.5kHz, CSHUNT = 1nF
1
11426-003
RIPPLE (mV rms)
100
M2.000µs
Figure 3. Output Voltage Ripple Before Low-Pass Filter
The low-pass filter (3 kΩ, 0.01 µF) between the AD598 output
and the AD8615 input has a −3 dB bandwidth of 5.3 kHz and
reduces the ripple to 2 mV p-p.
0.1
1
C2, C3, C4; C2 = C3 = C4 (µF)
10
11426-002
2.5kHz, CSHUNT = 10nF
0.1
0.01
Figure 2. Output Voltage Ripple vs. Filter Capacitance
The maximum number of rms counts that can be resolved can
now be calculated by dividing the full-scale output by the total
system rms noise.
With the low-pass filter installed between the output stage
of the AD598 and the input stage of the AD8615, data was
collected from the EVAL-CN0288-SDPZ evaluation board,
as shown in Figure 4.
Total RMS Counts = 5 V/0.4 mV = 12, 500
The effective resolution is found by taking the base 2 logarithm
of the total rms counts.
Effective Resolution = log2(12,500) = 13.6 Bits
Noise-free code resolution can be obtained by subtracting
2.7 bits from the effective resolution.
Noise-Free Code Resolution = Effective Resolution − 2.7 Bits
11426-004
= 13.6 Bits − 2.7 Bits
= 10.9 Bits
Figure 4. Screenshot of the CN-0288 Evaluation Software
The ripple from the AD598 was attenuated to 2 mV p-p, and the
system was able to achieve 11 bits of noise-free code resolution.
A complete design support package for this circuit note can be
found at http://www.analog.com/CN0288-DesignSupport.
Rev. A | Page 3 of 6
CN-0288
Circuit Note
Applications in Flight Control Surface Position Feedback
CIRCUIT EVALUATION AND TEST
Unmanned autonomous vehicles (UAVs), or drones, are playing
an ever-increasing part in the national security of the United
States. These high technology, complex aerial platforms are
controlled by a crew miles away and are multimission capable.
They include roles such as aerial reconnaissance, combat
weapons platforms, battlefield theater command and control
oversight, or unmanned in-flight refueling station.
This circuit uses the EVAL-CN0288-SDPZ circuit board and
the EVAL-SDP-CB1Z SDP-B system demonstration platform
controller board. The two boards have 120-pin mating connectors,
allowing for the quick setup and evaluation of the performance
of the circuit. The EVAL-CN0288-SDPZ contains the circuit to be
evaluated, and the EVAL-SDP-CB1Z (SDP-B) is used with the
CN-0288 evaluation software to capture the data from the
EVAL-CN0288-SDPZ.
The complex systems employed on UAVs use a myriad of
electronic sensors for precise control and feedback. To control
the altitude (pitch, roll, and yaw) of the UAV, actuators are used to
exert forces on the flight control surfaces. The precise measurement
of the position of these actuators is crucial in maintaining the
proper flight of path.
The sensors used to measure actuator position need to meet
three essential criteria: high accuracy, high reliability, and light
weight. All three of these attributes are found in the LVDTs
designed by Measurement Specialties, Inc.
Synchronous Operation of Multiple LVDTs
In many applications, such as multiple gaging measurement, a
large number of LVDTs are used in close proximity. If these
LVDTs operate at similar carrier frequencies, stray magnetic
coupling can cause beat notes to be generated. The resulting
beat notes may interfere with the accuracy of measurements
made under these conditions. To avoid this situation, all LVDTs
operate synchronously.
The EVAL-CN0288-SDPZ evaluation board can be configured
to have one master oscillator between two LVDTs by populating
Jumper JP1, JP2, and JP4 with a shorting jumper and leaving
JP3 unpopulated. Each LVDT primary is driven from its own
power amplifier, and, thus, the thermal load is shared between
the AD598s.
Equipment Needed
The following equipment is needed:
•
•
•
•
•
•
A PC with a USB port and Windows® XP (32 bit),
Windows Vista®, or Windows 7
The EVAL-CN0288-SDPZ circuit board
The EVAL-SDP-CB1Z SDP-B controller board
The CN-0288 evaluation software
The EVAL-CFTL-6V-PWRZ dc power supply or
equivalent 6 V/1 A bench supply
Measurement Specialties, Inc., E-100 Economy Series LVDT
(EVAL-CFTL-LVDT)
Getting Started
Load the evaluation software by placing the CN-0288 evaluation
software into the CD drive of the PC. Using My Computer, locate
the drive that contains the evaluation software.
Functional Block Diagram
See Figure 1 for the circuit block diagram and the EVAL-CN0288SDPZ-PADSSchematic.pdf file for the complete circuit schematic.
The PDF file can be found in the CN-0288 Design Support
Package.
COMMON VARIATIONS
The components selected were optimized for a maximum 5 V
unipolar output from the AD598; however, other combinations
can be substituted.
MEASUREMENT
SPECIALTIES, INC.
E-100 ECONOMY
SERIES LVDT
EVAL-CFTL-LVDT
Other suitable single-supply amplifiers are the AD8565 and
AD8601. These amplifiers are suitable replacements for the
AD8615 because they have input overvoltage protection and the
ability to swing rail-to-rail at both the input and output. If dualsupply operation is required, the ADA4638-1 or ADA4627-1 is
suggested.
EVAL-CFTL-6V-PWRZ
6V WALL WART
PC
J4
USB
120
PINS
J8
EVAL-CN0288-SDPZ
BOARD
If the AD598 outputs ±10 V bipolar signals, the AD7321 is
suggested. The AD7321 is a 2-channel, bipolar input, 12-bit ADC
that can accept true bipolar analog input signals as large as ±10 V.
Rev. A | Page 4 of 6
CON A
EVAL-SDP-CB1Z
SDP-B BOARD
Figure 5. Test Setup Block Diagram
11426-005
J3
Circuit Note
CN-0288
Setup
Connect the 120-pin connector on the EVAL-CN0288-SDPZ to
the CON A connector on the EVAL-SDP-CB1Z (SDP-B). Use
nylon hardware to firmly secure the two boards, using the holes
provided at the ends of the 120-pin connectors. With power to
the supply off, connect a 6 V power supply to the +6 V and GND
pins on the board. If available, a 6 V wall wart can be connected to
the barrel connector on the board and used in place of the 6 V
power supply. Connect the USB cable supplied with the EVALSDP-CB1Z to the USB port on the PC. Do not connect the USB
cable to the Mini-USB connector on the EVAL-SDP-CB1Z at
this time.
Test
Apply power to the 6 V supply (or wall wart) connected to the
EVAL-CN0288-SDPZ. Launch the evaluation software and
connect the USB cable from the PC to the Mini-USB connector on
the EVAL-SDP-CB1Z.
When USB communications are established, the EVAL-SDPCB1Z can send, receive, and capture parallel data from the
EVAL-CN0288-SDPZ.
Figure 6 shows a photo of the EVAL-CN0288-SDPZ connected
to the EVAL-SDP-CB1Z. Information regarding the EVALSDP-CB1Z can be found in the UG-277 User Guide.
Information and details regarding test setup and calibration,
and how to use the evaluation software for data capture can be
found in the CN-0288 Software User Guide.
Connectivity for Prototype Development
The EVAL-CN0288-SDPZ is designed to use the EVAL-SDPCB1Z; however, any microprocessor can be used to interface
to the I2C 2-wire serial interface of the AD7992. In order for
another controller to be used with the EVAL-CN0288-SDPZ,
software must be developed by a third party.
There are existing interposer boards that can be used to interface to
the Altera and Xilinx field programmable gate arrays (FPGAs).
The BeMicro SDK board from Altera can be used with the
BeMicro SDK/SDP interposer using nios drivers. Any Xilinx
evaluation board that features the FMC connector can be used
with the FMC-SDP interposer board.
The EVAL-CN0288-SDPZ is also compatible with the Digilent,
Imod interface specification.
11426-006
A photo of the system is shown in Figure 6.
Figure 6. The EVAL-CN0288-SDPZ Board Connected to EVAL-SDP-CB1Z (SDP-B) Board and Measurement Specialties, Inc., E-100 Economy Series LVDT
Rev. A | Page 5 of 6
CN-0288
Circuit Note
LEARN MORE
Data Sheets and Evaluation Boards
CN-0288 Design Support Package:
http://www.analog.com/CN0288-DesignSupport
CN-0288 Circuit Evaluation Board (EVAL-CN0288-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
SDP-B User Guide
AD598 Data Sheet
Ardizzoni, John. A Practical Guide to High-Speed Printed-CircuitBoard Layout. Analog Dialogue 39-09, September 2005.
AD7992 Data Sheet
MT-004 Tutorial, The Good, the Bad, and the Ugly Aspects of
ADC Input Noise—Is No Noise Good Noise?, Analog Devices.
ADP1613 Data Sheet
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”, Analog Devices.
AD8615 Data Sheet
ADP7104 Data Sheet
REVISION HISTORY
MT-035, Op Amp Inputs, Outputs, Single-Supply, and Rail-toRail Issues, Analog Devices.
3/14—Rev. 0 to Rev. A
Changes to Synchronous Operation of Multiple LVDTs Section ..... 4
MT-036 Tutorial, Op Amp Output Phase-Reversal and Input
Over-Voltage Protection, Analog Devices.
3/13—Revision 0: Initial Version
MT-068 Tutorial, Difference and Current Sense Amplifiers,
Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
AN-1106 Application Note, An Improved Topology for Creating
Split Rails from a Single Input Voltage, Analog Devices.
E-100 Economy Series LVDT, Measurement Specialties, Inc.
The LVDT: construction and principle of operation, Technical
Paper, Measurement Specialties, Inc, 1000 Lucas Way,
Hampton, VA 23666.
Subminiature LVDTs Provide Accurate Flight Control Surface
Position Feedback on UAVs, Application Note, Measurement
Specialties, Inc, 1000 Lucas Way, Hampton, VA 23666.
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CN11426-0-3/14(A)
Rev. A | Page 6 of 6